1. Field of the Invention
The present invention relates to focus detection devices and image capturing apparatuses provided with the same.
2. Description of the Related Art
At present, image capturing apparatuses such as still cameras, video cameras, and so on generally include auto focus detection (AF) functionality. The contrast detection method and the phase-difference detection method are widely used as auto focus detection methods. Of these, the phase-difference detection method detects the distance to a subject (or a defocus amount) based on the phase difference between signals obtained from a pair of line sensors, using the principle of triangulation. The position of a focus lens, which is included in an imaging lens, is controlled based on the results of the detection, thus making it possible to focus the imaging lens on the subject.
Two systems are known as specific configurations for realizing the phase-difference detection method; one, known as the TTL system, uses a line sensor to receive light that has entered into the imaging lens, whereas the other, known as the external AF system, uses a line sensor to receive light that has entered through a different path than the imaging lens. The former is capable of detecting a defocus amount based on the phase difference between output signals from the line sensors, whereas the latter is capable of detecting the distance to the subject.
Furthermore, in recent years, focus detection devices that use line sensors in which pixels are arranged in a staggered manner, so as to improve the accuracy of auto focus detection through the phase-difference detection method, are being put into practical use. This line sensor in which the pixels are arranged in a staggered manner has a configuration in which multiple unit line sensors are arranged adjacent to each in the direction orthogonal to the alignment direction of the pixels and so that the pixel positions are shifted by 0.5 pixels in the alignment direction. Japanese Patent Laid-Open No. 2006-133515 proposes improving focus detection accuracy by using a line sensor in which the pixels are arranged in a staggered manner and carrying out phase-difference detection using a combination of or individual image signals obtained from the unit line sensors.
Meanwhile, with the external AF system, the optical system that carries out focus detection (that is, the focus-detection optical system) is provided in a different position than the optical system in which is formed the image of the captured subject (that is, the imaging optical system), and thus parallax between the two arises, resulting in a mismatch between the image capturing area and the focus detection area. Due to this parallax, the region that can be used as a focus detection region in the field of view of the imaging optical system is limited. To reduce the influence of such parallax, parallax adjustment is carried out during the manufacture of the image capturing apparatus, where the center of the field of view of the imaging optical system is caused to correspond to the center of the field of view of the focus-detection optical system having assumed a subject at a predetermined distance. Focus detection through the external AF system on a subject within the field of view of the imaging optical system is realized through such parallax adjustment.
However, in the case where a line sensor in which pixels are arranged in a staggered manner is used in the external AF system, and parallax adjustment is carried out so that the centers of the fields of view of the imaging optical system and the focus-detection optical system correspond, there are situations in which the focus detection accuracy deteriorates due to the angle of view (when variable) of the imaging optical system, the subject distance, and so on.
For example, consider a line sensor in which two unit line sensors are arranged adjacent to each other in two rows. In this case, if the center of the field of view of the imaging optical system and the center of the field of view of the focus-detection optical system are caused to correspond, the center of the field of view of the focus-detection optical system falls upon the border between the two unit line sensors. Generally speaking, images are captured so that the main subject that is to be focused on is located in the center of the field of view, and thus if the center of the field of view of the imaging optical system falls upon the border between the unit line sensors, the light of the image of the main subject will be received having been split between the upper and lower unit line sensors. If the angle of view of the imaging optical system is narrowed, the number of pixels in the unit line sensors corresponding to the field of view of the imaging optical system drops, which makes it easier for the focus detection accuracy to deteriorate; meanwhile, if the image of the main subject is received being split between the upper and lower unit line sensors, it is easier for the focus detection accuracy to deteriorate as well.